1. Field of the Invention
The invention relates to paper machine clothing useful for fabrication of dryer belts, employed in the dryer section of a papermaking machine, wet belts employed in the press section of such machines and forming wires which may be used on fourdrinier and cylinder machines and more particularly relates to such fabrics having controlled void volumes.
2. Brief Description of the Prior Art
On papermaking machines, endless woven belts are employed in various sections to carry the sheet or web of paper. For example, in the dryer section such belts carry the sheet of paper in close contact with the heated dryer cylinders. There are a wide variety of forms of the endless woven belts, some fabricated from metal and others from textile material such as cotton, cotton and asbestos or cotton, asbestos and synthetic fibrous or filamentous materials. The selection of a given material is dependent to some degree upon the use to which the fabric will be put, i.e.; as a forming fabric, dryer felt, etc. One form of belt commonly employed in the dryer section of a papermaking machine is referred to as a "screen" and is fabricated by weaving synthetic monofilaments or twisted multi-filaments together in an open weave. Although not subjected to any form of milling, and therefore not "felts" in the original sense of the term, these screen fabrics have also become known as "dryer felts". The endless belts are generally woven flat and the ends thereafter joined to form an endless belt. The weave selected may be a two or three layer weave of synthetic yarns such as multifilament, spun or monofilament yarns.
It will be appreciated that the screen type of "dryer felt" fabric is relatively open in design, resulting in a relatively high fabric permeability, i.e.; air permeability on the order of from about 70 to 700 CFM/sq. ft. at 1/2" water. Such fabrics permit free vapor passage through the fabric during operation of the papermaking machine. In some machines, such a high permeability may not be desirable, although other characteristics of the screen are advantageous. To lower the fabric air permeability in dryer screens it has heretofore been suggested that internal fabric interstices or voids between lengthwise and cross-wise yarns in the multi-layers be at least partially filled with "stuffer picks" or spun yarns; see British Pat. No. 1,207,446. The stuffer picks form internal baffles in the fabric and reduce overall permeability of the fabric. However, the presence of the spun yarn stuffer picks also increases the moisture retention of the fabric. Over a period of time this may reduce the drying efficiency of a given paper machine operation employing this kind of dryer felt.
The dryer felt fabric of the present invention obviates a number of problems associated with prior art dryer fabrics, particularly of the screen type. The fabrics of the invention are characterized by low permeability, i.e.; on the order of from 5 to 300 CFM/sq. ft. at 1/2" water. However, they do not retain moisture to any significant degree above that observed for screens fabricated without inclusion of "stuffer picks". The fabrics of the invention are advantageously used to fabricate "dryer felts" for papermaking machines.
Although the method of the invention is particularly advantageous for controlling the void volume in dryer felts, it may also be used to control void volume in wet belts, i.e.; true papermaker's felts and in forming wires. Such uses will be discussed more fully hereinafter.